1. Field of the Invention
The present invention relates to interference cancellation techniques, and more particularly, to digital interference techniques at radio frequencies.
2. Description of the Prior Art
The problem of interference in RF receivers is well known. One approach is to try to filter the interferers; the other is to actively cancel them by generating a signal corresponding to each interferer and subtracting it from the input. In the former technique, it is possible that signal components are lost in the filter, since these generally cannot distinguish between the interferer and the signal of interest if it has components within the filter stop band. The latter has the ability to model the interferer, and therefore remove only the interferer without substantial loss of signal. In order to effectively cancel an interferer, its characteristics must be well known, and, in the event of a dynamic interferer, its temporally changing characteristics must also be modeled, since inappropriate attempts at cancellation can increase interference.
One can describe three common classes of interferers (see FIG. 1). Co-site interference comes from one's own transmitter, where one typically has direct access to the interferer. Another type is interference that is of a known signal type, which may be present to different degrees at different times (such as a radar source or a simple jammer). This can be generated from a signal template, with appropriate gain, delay, and parametric adjustments. A third type is one that is a complex, unknown signal, which must be carefully measured in real-time to be actively cancelled.
There are many prior patents on active interference reduction. A few of these might include the following: U.S. Pat. No. 5,729,829 Talwar and Fitzgerald (American Nucleonics Corp), Interference mitigation method and apparatus for multiple collocated transceivers, issued 1998; U.S. Pat. No. 6,693,971 Kowalski (BAE Systems), Wideband Co-Site interference reduction apparatus, issued 2004; and U.S. Pat. No. 7,058,368 Nicholls and Roussel (Nortel), Adaptive feedforward noise cancellation circuit, issued 2006, each of which is expressly incorporated herein by reference.
Known attempts to cancel interferers either employ analog signals, or use digital baseband signals with analog upconversion or carrier envelope modulation. Indeed, in the case of narrow-band interferers, digital baseband signal generation with upconversion and amplitude and phase adjustment to obtain optimum cancellation produces generally acceptable results (see FIG. 3).
Tolerating high levels of interference, including those from co-located RF transmitters, is a longstanding problem for building wideband and sensitive naval signals intelligence (SIGINT) and communication receivers. The presence of unwanted signals or interferers reduces the usable spectrum and hence the dynamic range of the receiver. The traditional method of dealing with the interference problem is to use band-stop or notch filters to excise the interferers from the band-of-interest. This approach, employing analog RF components, works well when there are a small number of fixed interferers spectrally separated from the signals of interest (FIG. 2A). It does not work well when the interferers are numerous, overlap the signal bands of interest, or change their spectral locations rapidly (see FIG. 2B).
A recent approach to broadband radio receivers is to directly digitize an entire broad band at a very high sampling rate, followed by digital processing to select signals of interest and reject interferes. Such a Digital-RF™ (Hypres Inc., Elmsford, N.Y.) receiver enables flexible, reconfigurable reception of multiple signals across the band. However, the presence of large interferers within this broad band requires an analog-to-digital converter (ADC) with extremely high linear dynamic range. In principle, digital filters may be used to remove interferers in the digital domain. In practice, if the interferers are too large, they will saturate the ADC, limiting the dynamic range available to the signal. In addition, small amounts of nonlinearity in the analog signal processing chain can result in intermodulation products and harmonics. If one or more interferers are strong, and the signal has a wide bandwidth, the pattern of spurious signals (so-called “spurs”) can interfere with processing of a signal of interest. So it is necessary in the case of large amplitude interferers to cancel or substantially reduce the interferers early in the analog signal processing chain, before the ADC, to avoid non-linear effects such as saturation and intermodulation. As used herein, it is understood that a quantizer or digitizer is a non-linear component of a circuit, and therefore that the phrase non-linear component includes such digitizers and quantizers. It is also understood that real implementations of analog to digital converters are typically saturable, that is, having a response dependent on a past history, especially if the signal presented exceeds a saturation level, and thus appears to respond non-linearly on that basis.
Typically, a Digital-RF™ receiver will have a digital sample rate in excess of a Nyquist rate of a significant signal represented therein. That is, the Digital-RF™ signal is oversampled with respect to the corresponding analog representation. At radio frequencies, this may involve very high data rates. Advantageously, such digital data generation and handling capability is available from superconducting electronics, though other technologies may also be employed.